The present invention relates to fan ducts and, more particularly, to a transition duct between the air mover, i.e., centrifugal fan, and air consumer, i.e., system being served by the air mover.
In many centrifugal fan applications space is limited and yet may be a critical design consideration. Most applications require that the fan be connected to an outlet transition duct, the length and design of which can greatly affect the overall performance of the overall system fan efficiency. Traditionally, this duct has been of considerable length and of a symmetrical configuration in the cross section perpendicular to the air flow direction.
In the Fan Application Manual of the Air Moving & Conditioning Association, the section at pages 22-23 indicates that maximum efficiency shown in the rating tables supplied by a manufacturer "will not be achieved unless a comparable (transition) duct is included in the system design ...For 100% (velocity) recovery the (transition) duct should extend at least two and one half equivalent duct diameters." The equivalent duct diameter for a rectangular duct is determined by a duct's height "a" and width "w" and equals ##EQU1## For normal air outlet velocities of about 4,000 feet per minute (1,219 meters per minute) transition duct length should be as long as 4 equivalent duct diameters, according to the manual. The loss of fan efficiency can be as much as 50% if an elbow or similar bend in the duct were very close to the fan outlet.
The conventionally recommended transition duct lengths have resulted in serious design problems and limitations on certain fan applications, as when fans are close-coupled to cooling towers. To obtain optimal fan efficiency, space has to be allotted for the recommended transition duct length. For example, in certain cooling tower applications, it is desirable to have the cooling tower as low and narrow as possible. Where a centrifugal fan is used in the cooling tower to either intake air into the cooling tower or to exhaust air from the cooling tower, it is necessary to allot extra height and width to the cooling tower to accommodate the recommended transition duct length.